1. Field of the Invention
This invention relates generally to a method and apparatus for printing solder paste for printed circuit boards and, more particularly, to an apparatus for indicating a predetermined snap-off between a circuit board and a mask used during the solder paste printing process.
2. Description of the Prior Art
During the conventional solder paste printing process, solder paste is used to form the interconnections between electronic components and the circuit board. The solder paste is preferably supplied in an amount sufficient to provide both electrical and mechanical connections without producing bridging.
The basic materials used in the solder paste printing process are masks comprising a stencil or screen; squeegee; solder paste and a circuit board. Variables besides these basic materials that affect print quality are printing machine parameters or settings. Some of the machine settings or parameters comprise the following items, squeegee speed and pressure, snap-off and the relative parallelism of the circuit board, stencil/screen and the squeegee. For example, the squeegee shape and hardness will affect the end result of the solder paste printing process. Also, the type of solder paste used including its viscosity, tackiness, particle size and percent metals will effect the end result of the process.
Of the two methods of printing solder paste, stencil printing is presently the most prevalent method used though screen printing is still used in many instances. Stencil printing allows the use of solder paste with larger particle size and a higher viscosity. However, one of the more critical setup requirements for the stencil printing equipment is snap-off. Snap-off is defined as the distance between the circuit board and the stencil. One of the benefits of proper snap-off is better separation of the stencil from the circuit board after the solder paste has been deposited. Also, too much snap-off makes it difficult to maintain the X-Y alignment of the stencil and smearing becomes more of a problem. Snap-off is usually small, approximately 0.040 inches. However, as a rule of thumb the maximum snap-off used should not exceed ten times the stencil thickness.
During the solder paste printing process, the stencil completely covers up the entire circuit board leaving no access from the sides or below to determine if the proper snap-off has been achieved. One of the ways that snap-off is measured is through a hole in the stencil. Particularly, a pin is stuck down through the stencil hole until it engages the circuit board, marked and the distance on the marked pin is then measured. If the desired snap-off is not achieved, then the stencil or the circuit board is moved and the measurement taken again. If the desired snap-off is still not achieved, the stencil or the circuit board would be moved again and the measurement would be taken again.
Another method for setting the snap-off is designed for use with the Fuji GSP-II screen printer manufactured by Fuji Machine Manufacturing Company, Ltd. of Japan. In this method, the circuit board is positioned onto the vacuum plate of the GSP screen printer. The stencil is secured to a frame on the GSP screen printer. The frame, positioned above the vacuum plate, is raised. The vacuum plate holding the circuit board is then lowered to its lowest position by using the "snap-off" control hand crank. A bare board with a predetermined height equal to the desired snap-off distance of 0.040" is then centrally positioned on top of the circuit board. The frame with the stencil is lowered. The snap-off control hand crank is turned to move the vacuum plate upward until the top of the bare board "just" touches the underside surface of the stencil. This touching of the board to the stencil can be seen through the holes in the stencil. If the stencil begins to deflect upward the snap-off hand crank is backed off (moving the vacuum plate downward) to lower the circuit board. This position is then set into the memory of the GSP screen printer. The frame with stencil is then raised, the bare board removed from the circuit board, and the frame is returned to its set position. Subsequently the squeegee is used to apply the solder paste and the electronic components are then placed onto the paste. The circuit board with the paste deposited on it is then sent through an infra-red oven where it becomes liquidous and solders the electronic components in place.
An alternative to the above FUJI method for setting the snap-off is to use a 0.040 inch thick plastic 5".times.5" shim gauge with one side painted red instead of the above discussed bare board. This 5".times.5" red shim gauge is centrally positioned on the circuit board where the red painted side can be seen through the stencil holes. Both the bare board and red shim guage methods of setting the snap-off require that the entire board/guage touch the bottom of the stencil not just the corners since the board/guage may be slightly bowed. In some cases the operator is unable to determine when the shim gauge is fully engaging the stencil, making the accuracy less than desired. An easier and more accurate method to measure snap-off is desired in the industry.